Corrosion protection

ABSTRACT

A SOURCE OF FREE ELECTRONS, SUCH AS IS SUPPLIED BY THE GRID OF A CONDUCTING VACUUM TUBE, IS CONNECTED BY A SINGLE ELECTRICAL CONDUCTOR TO A METALLIC STRUCTURE TO BE PROTECTED FROM CORROSION. NO RETURN PATH FOR ELECTRONS FROM THE METALLIC STRUCTURE TO THE SOURCE IS PROVIDED AND IT IS NOT REQUIRED THAT THE PROTECTED STRUCTURE BE EARTH GROUNDED.

Feb. 2, 1971 V HOOD 3,560,359

CORROSION PROTECTION Fil ed May 5 1969 I N VE N TOR Q0/5 20 A. #000HTTOFA/[Ki United States Patent O 3,560,359 CORROSION PROTECTION RichardL. Hood, 10333 Dolecetto, Rancho Cordova, Calif. 95670Continuation-impart of application Ser. No. 384,376, July 22, 1964. Thisapplication May 5, 1969, Ser. No. 825,121

Int. Cl. C231? 13/ U.S. Cl. 204-147 7 Claims ABSTRACT OF THE DISCLOSUREA source of free electrons, such as is supplied by the grid of aconducting vacuum tube, is connected by a single electrical conductor toa metallic structure to be protected from corrosion. No return path forelectrons from the metallic structure to the source is provided and itis not required that the protected structure be earth grounded.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of application Ser. No. 384,376, filed July 22,1964, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to corrosion protection and, more particularly, to apparatus forprotecting a metallic structure from corrosion by connecting thestructure to a source of electrons.

Review of the prior art and the problems addressed by the invention Theprocess of corrosion of a metallic structure is essentially anelectrolytic process in that corrosion involves a loss of electrons fromthe structure, and the presence of an electrolyte is required beforecorrosion will occur. The loss of electrons from the metal structure isusually localized in that some areas of the structure become positivewith respect to other areas. Those areas which are anodic with respectto the remainder of the structure corrode fastest, although corrosionoccurs at all points of the structure which are in contact with theelectrolyte. Anodic areas are deficient in electrons relative to otherareas of the corroding structure.

Many methods have been used with varying degrees of success tocounteract electron loss from a metallic structure in contact with anelectrolyte, thereby to prevent or inhibit corrosion. The simplestmethod is to cover the structure with a protective coating, such aspaint, which prevents contact between the structure and the electrolyte,thereby preventing one area of the structure from becoming anodic withrespect to another area of the structure in the presence of theelectrolyte. One basic method of corrosion protection requires that thewhole of the structure to be protected be made cathodic with respect toa sacrificial electrode. For example, the protected structure may bemade the cathode of a galvanic cell in which the anode, connected to thestructure by an electrical conductor, is more active in the galvanicseries than the metal protected; a specific example is provided whereZinc plates are bolted to the hull of a ship proximate a bronzepropeller to be anodic relative to the propeller in the presence ofwater (normally salt water but frequently fresh water) and to corrode inpreference to the more expensive propeller. An electrolyte, such aswater with impurities or moist soil, serves as the return path forcurrent flowing in such a cell, and the cathode is protected at theexpense of the anode. Another related method requires that a directcurrent potential be impressed upon the protected structure to renderthe entirety of the protected structure cathodic relative to earthground or to a sacrificial anode which, in the case of an impressedpotential, may be defined by a metal occupying a position lower in thegalvanic series than the metal of the protected structure.

It has long been believed that a complete circuit must be made betweenthe anode and the cathode in any cathodic protection system so thatcurrent may flow. Considerable attention has been directed to the amountof current which must flow between the anode (either sacri ficial orthat defined by an impressed potential) and the protected structure inorder to completely overcome all traces of local anodization of thestructure.

A more complete description of prior efforts in the area of cathodicprotection are set forth in Corrosion Hand book" by Herbert H. Uhlig,John Wiley & Sons, 1948, page 923 et sequor, and in Corrosion andCorrosion Control by Herbert H. Uhlig, John Wiley & Sons, 1963, chapter12.

PROPOSED THEORY OF THE INVENTION The method and apparatus of thisinvention are based upon the discovery that the problem of electrolyticcorrosion may be overcome merely by connecting the structure to beprotected with a source of free electrons. Neither a sacrificial anodenor a return conductive path from the protected article to the source ofelectrons is required; this feature of the invention is distinguishedfrom prior methods of corrosion protection or inhibition wherein asacrificial anode is connected by means of an electrical conductor tothe protected structure and a return path is made from the structure tothe anode through the electrolyte. It has been found that the inventionis effective in protecting all parts of a structure, such as a wellcasing or the like, even though the structure itself has parts which areelectrically insulated from one another.

The precise mechanisms which are at work between the protected structureand an electrolyte during operation of this invention are not known withcertainty. It is believed, however, that one area of the structure maytend to become anodic with respect to another area of the structure byreason of the presence of ions in the electrolyte contacting thestructure. That is, if a positive ion moves into proximity to an atom inthe metallic structure, the metal atom may tend to lose a valenceelectron to the ion and thereby become chemically active suificient toparticipate in the corrosion process which is generally agreed to be anelectrochemical process; it will be understood, too, that the atom, byloss of an electron, becomes positive and renders the area of thestructure in its vicinity anodic with respect to other areas of thestructure Where similar events have not occurred. The longer the activeatom remains deficient in a valence electron (i.e., remains chemicallyactive), the greater the probability that the atom will participate inthe electrochemical corrosion process. This invention reduces theprobability that a chemically active atom (an atom missing one or morevalent electrons) will participate in the corrosion process by making itpossible for such atom to replace the lost electrons from within themetallic structure itself rather than by reacting chemically orelectrochemically with the electrolyte. This electron replacementprocess is accomplished in such a way that another atom of the metallicstructure to be protected is not required to give up an electron andthereby become susceptable to participation in the corrosion processwith the electrolyte. The corrosion inhibition or prevention effect ofthis invention is directly related to the speed with which lostelectrons are replaced.

The structure to be protected is provided with an excess of electrons,i.e., is connected conductively to a source of free electrons which arenot associated with an atomic nucleus. Therefore, an alternate theoryrecognizes that when a positive ion in the electrolyte moves intoproximity to an atom in the metallic structure, a discrete amount offorce must be exerted on the valence electrons of the atom to overcomethe forces acting on such electrons within the atom before one of suchatoms can be lost to the ion. The force required to draw an excesselectron from the metallic structure is less than the force required todraw an electron from an atom. Thus, the electrolyte satisfies itselectron demands from the source of free electrons via the metallicstructure more readily than from the valence electrons of the atoms inthe structure, with the result that a significantly diminished number ofatoms in the structure become involved in the corrosion process.

It is believed that these alternate theories of the operation of thepresent invention are compatible with the principles at work in theprior corrosion prevention and inhibition methods described above. Thesacrifical anode method relies upon there being some other metal incontact with the electrolyte to respond to the presence of an ion in theelectrolyte faster than the atoms of the principal metal of interest,i.e., the metal to be protected from corrosion. In the sacrifical anodemethod, the anode metal is higher in the electromotive table than theprincipal metal so that a galvanic cell is produced and negative ions inthe electrolyte prefer to gather at the sacrificial metal rather than atthe principal metal. The impressed potential method renders the entirebody of the principal metal cathodic relative to ground or to some otherconductive body so that even though isolated atoms of the principalmetal may respond to ions in the electrolyte and become anodic relativeto other areas of the same metal, such atoms are still cathodic relativeto the electrolyte.

SUMMARY OF THE INVENTION Generally speaking, the present inventionprovides apparatus for preventing corrosion of a metallic structure. Theapparatus comprises a source of free electrons and means for conductingelectrons from the source to the metallic structure to be protected sothat electrons lost from the structure to an electrolyte in contact withthe metallic structure are replaced from the source more readily thanfrom the electrolyte. The source, except for the conducting meansconnected from the source to the metallic structure, is electricallyisolated from the structure and from earth ground.

As used herein the term source of free electrons encompasses a source inwhich electrons are not constrained within a crystalline lattice orreleased by an electrochemical reaction. A preferred source is the gridof a conducting triode-type vacuum tube.

BRIEF DESCRIPTION OF THE DRAWINGS The above mentioned and other featuresof this in vention are more fully set forth in the following detaileddescription taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an apparatus according to thisinvention; and

FIG. 2 is a schematic diagram of a second embodiment of this invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS OF THE INVENTION FIG. 1illustrates a well casing pipe extending vertically through a geologicalformation 11. The pipe is exemplary of a metallic structure which is tobe protected from corrosion. FIG. 1 also illustrates a corrosionprotection apparatus 12 which is essentially a source of free electronsin combination with means for maintaining the source. The protectionapparatus is electrically connected to a pipe 10 by a conventionalelectrical conductor 13.

Corrosion protection apparatus 12 includes an isola tion transformer 15having a primary winding 16 and a secondary winding 17, the windingspreferably having a turns ratio of 1:1. The primary winding is connectedto a source of alternating current such as a 117 volt AC source. Theisolation transformed provides that direct current does not flow betweenthe primary and secondary thereof, but that a voltage is generated inthe secondary only by reason of the flux linkage between the primary andsecondary. Secondary winding 17 is connected across a rectifier 19,shown in FIG. 1 as a diode vacuum tube 20, although any suitablerectifier may be used. Rectifier 19 preferably is the diode portion of atriode-connected 117L7 vacuum tube. A filter capacitance 21, preferablyhaving a value of about 20 microfarads where a 117L7 tube is used, isconnected in series between the transformer secondary and the cathode ofthe diode across a load resistance formed by series resistors 28 and 31.Where a 117L7 tube is used in apparatus 12, resistors 28 and 31 may havevalues of 47 and 6800 ohms, respectively. Thus the rectifier acts as apower source of a direct potential developed across the load resistance.

A triode 22 having a cathode 23, a grid 24 and an anode 25 is powered bythe direct current source provided by the diode 19 and preferably is thetriode portion of a triode-connected 117L7 vacuum tube. A loadimpedance, in the form of an anode resistor 27 in series with theresistance 28, is coupled across the anode and cathode of triode 22.Resistor 27 may have a value of 560 ohms. A common terminal 29 betweenresistors 27 and 28 is connected to the rectifying circuit between thediode cathode and capacitance 21, and a return 30 from the triodecircuit to the rectifier circuit is provided from the opposite end ofresistor 28 to the rectifier circuit between capacitance 21 andisolation transformer secondary winding 17. This arrangement provides alow plate load for triode 22 relative to the internal impedance of thetriode. The resistor 31 is provided in the return connection from thetriode to the rectifier circuit and the grid 24 is connected by a largegrid resistor 32 to the junction between the resistors 28 and 31 toimpart a small amount of self bias to the triode. The resistors 31 and32 are not essential but protect the triode from being overloaded. Inview of the low plate load and small self bias of the triode, the triodeis heavily conducting and grid 24 is maintained in a heavy stream offree electrons.

As noted above, grid 24 is connected to the well casing 10 by aconductor 13. The grid functions as means for collecting free electronsfrom the electron stream flowing between the cathode and anode of thetriode. As incipient corrosion is manifested at the casing by the lossof an electron from a given area of the casing surface, another electronis immediately withdrawn from the electron stream to take its place. Agiven replacement electron may be derived from the surrounding metal,but the number of electrons originally present in the casing ismaintained by connection of the casing to the free electron collector sothat anodization of the casing is prevented and corrosion is inhibited.

There is no direct connection of the corrosion protector circuitry toground and preferably the circuitry is housed in a completely insulatedenclosure. In other words, but for conductor means 13 which extends onlyfrom grid 24 to casing 10, an open electrical circuit exists between thecorrosion protection circuitry and pipe 10 so no return path forelectrons from pipe 10 to this circuitry is provided.

Electrons flow from grid 24 to casing 10 via conductor 13 only when ademand for electrons exists at the casing by reason of incipientcorrosion. Because no impedance, other than the impedance of conductor13 itself, is present in conductor 13 between the grid and the casing,the grid is at the same potential as the casing. The electron flow whichexists in conductor 13 is very small and is not a current in theconventional sense because no return path exists from the casing back tothe circuitry of apparatus 12. The absence of current flow back toapparatus 12 from casing 10 has been confirmed by coupling a specialinfinite input impedance vacuum tube voltmeter across the input andoutput terminals of transformer 15; no direct current potential ofeither polarity was detected.

FIG. 2 shows a corrosion protector 35 connected to a gasoline storagetank '36, the tank being another example of a structure which is to beprotected from corrosion. The corrosion protector is comprised of abattery 37 connected in series with a load impedance 38, the battery andload impedance being connected across a triode 39 having a grid 40connected to the storage tank by a single electrical conductor 41. Noreturn path is provided from tank 36 to protector 35.

The source of electrons to which the protected structure is connected inthe practice of this invention must be a source of electrons which arefree of chemical or interatomic bonds. A storage battery is not anacceptable source since a battery produces electrons only when a circuitis completed across the terminals of the battery. Also, the electronsassociated with a storage battery are produced by electrochemicalreactions, and the threshold forces involved in such reactions are ofthe same order of magnitude or greater than the forces associated withremoval of an electron from casing 10, for example, by a contactingelectrolyte, with the result that there is no preference by theelectrolyte for a battery electron as opposed to an electron from thecasing. Similarly, it is believed that conducting transistor or othersolid state device may not be an effective source of free electronsbecause, in such devices, electrons move in a crystalline environmentand may not be readily removable from such environment.

*It has been found that merely connecting the protected structure to asource of free electrons, such as the grids of triodes 22 and 39,prevents corrosion in the structure. In a test of apparatus according tothis invention, two identical steel rods were buried in moist soil for aperiod of five days. One of the rods was connected to a corrosionprotector according to this invention. After five days it was found thatthe rod connected to the grid of the corrosion protector bore no traceof corrosion whereas the other rod was severely corroded. In subsequenttests, the circuit of FIG. 1 has shown the ability to control corrosionof oil pipe lines for indefinite periods with no apparent oxidizing,scaling or pitting of the metal pipe.

In another test it was determined that an electron flow exists betweenthe corrosion protection apparatus and the protected article byinserting a high resistance megohms) in series with the grid of theapparatus. A small voltage drop was detected across the resistance withthe use of a very high impedance meter.

The invention described above has the feature that no sacrificial anodeis provided and not only is no provision made for a return path forelectron flow from the protected structure to the source of freeelectrons, but such a return path is intentionally eliminated. Apparatusprovided by this invention is extremely economical to 1,186.

A single corrosion protector may be connected to several protectedstructures if desired.

What is claimed is:

1. Apparatus for inhibiting corrosion of a metallic structure comprisinga source of free electrons, and means for conductively connecting thesource to a metallic structure in which corrosion is to be inhibited forflow of electrons from the source to the structure, the source but forthe connecting means being conductively isolated from the structure andearth ground, the apparatus being characterized by the absence of ametallic return path from the metallic structure in which corrosion isto be inhibited to the source of free electrons.

2. Apparatus according to claim 1 wherein the source comprises a vacuumtube having a cathode, an anode, and a grid disposed between the anodeand the cathode, and means for energizing the tube to produce a flow ofelectrons between the cathode and anode of the tube, the connectingmeans being connected between the structure and the grid of the tube.

3. Apparatus according to claim 2 characterized by the absence of aneffective conductive connection from the structure to the anode or thecathode of the tube.

4. Apparatus according to claim 2 wherein the tube energizing means iseffectively conductively isolated from earth ground.

5. Apparatus according to claim 2 wherein the energizing means comprisesan isolation transformer having a primary winding adapted for connectionto a source of alternating current, means for rectifying a signaldeveloped across the secondary winding of the transformer, and means forapplying the rectifying means output across said anode and cathode.

6. Apparatus for inhibiting corrosion of a metallic structure comprisinga source of free electrons, and means for conductively connecting thesource to the structure for flow of electrons from the source to thestructure and arranged to define an open electrical circuit between thesource and the structure, the apparatus being characterized by theabsence of a metallic return path from the metallic structure in whichcorrosion is to be inhibited to the source of free electrons.

7. A method for protecting a metallic structure from corrosioncomprising the steps of providing a source of free electrons, andsupplying electrons to the structure from the source via a singleconductor connected from the source in the absence of a return path forflow of electrons from the structure to the source.

References Cited UNITED STATES PATENTS 2,021,519 11/1935 Polin 204-1 96TA-HSUNG TUNG, Primary Examiner US, Cl. X.R. Z041.96

